1. Field of the Invention
The present invention relates in general to the field of hydrocarbon production, and in particular, to methods related to mapping the size and shape of hydraulic fractures in hydrocarbon reservoirs.
2. Description of the Related Art
Hydraulic fractures are frequently employed to improve reservoir contact and production rates in the oil and gas industry. Hydraulic fracturing has been used for over 60 years in more than one million wells. Hydraulic fracture stimulation is commonly applied to wells drilled in low permeability reservoirs. An estimated 90% of the natural gas wells in the United States use hydraulic fracturing to produce gas at economic rates. Successful hydraulic fracturing is generally considered vital for economic production of natural gas from shale beds and other ‘tight gas’ plays.
A hydraulic fracture is formed by pumping a fluid into the wellbore at a rate sufficient to increase the pressure downhole to a value in excess of the fracture gradient of the formation rock. The pressure causes the formation to crack, allowing the fracturing fluid to enter and extend the crack further into the formation. To keep this fracture open after the injection stops, a solid proppant is added to the fracture fluid. The proppant, which is commonly sieved round sand or other porous material, is carried into the fracture. This sand is chosen to be higher in permeability than the surrounding formation, and the propped hydraulic fracture then becomes a high permeability conduit through which the formation fluids can flow to the well.
Determining the size and orientation of completed hydraulic fractures is quite difficult, expensive, and inaccurate. Accordingly, the inventors have recognized that improved means are sorely needed. Existing methods which employ tiltmeters or microseismic detectors are used despite their limitations because some information, even imperfect information is valuable. Tiltmeter arrays, deployed on the surface or down a well, for example, provide a technology for monitoring the fracture geometry. The tiltmeters measure the horizontal gradient of the vertical displacement with great precision (up to one nanoradian), and an array of tiltmeters properly situated over a reservoir can be used to extract the surface deformation that is taking place because of processes occurring deep underground. With microseismic monitoring microseismic activity is measured by placing an array of geophones in a nearby wellbore or at the surface. By mapping the location of small seismic events that are associated with the growing hydraulic fracture during the fracturing process, the approximate geometry of the fracture can be inferred. The microseismic monitoring relies upon the detection of individual microseismic events associated with discrete fracture opening events, which can be located in three dimensions by triangulation, which is based on comparing acoustic arrival times at various sensors in a receiver array.
The distance that rock faces are separated during a hydraulic fracture is called the fracture width. Practical fracture widths range from about one millimeter up to about one centimeter. The sands, or similar materials, are used to “prop” open hydraulic fractures are, therefore, typically about one millimeter in diameter or less. Accordingly, recognized by the inventors is that there exists some significant physical constraints on mapping devices which would be deployed within a hydraulic fracture. For example, recognized by the inventors is that any transponders to be used for mapping hydraulic fractures and reservoir parameters must be able to physically fit into the fracture, not just adjacent the opening, but deeply therein, and therefore, should not be not more than about one millimeter in at least one dimension, to help ensure passage.
The use of conventional radio-frequency identification (RFID) transponders was explored. RFID is a technology that uses communication via electromagnetic waves to exchange data between a terminal and an object such as a product, animal, or person for the purpose of identification and tracking. Some tags can be read from several meters away and beyond the line of sight of the reader. RFID involves readers (also known as interrogators) and transponders (also known as tags). Most RFID tags contain two primary components. The first is an integrated circuit for storing and processing information, modulating and demodulating a radio-frequency (RF) signal, and other specialized functions. The second is an antenna for receiving and transmitting the signal. There are three types of RFID tags: passive RFID tags, which have no power source and require an external electromagnetic field to initiate a signal transmission; active RFID tags, which contain a battery and can transmit signals once an external source (‘Interrogator’) has been successfully identified; and battery assisted passive (BAP) RFID tags, which require an external source of sufficient power to “wake up” the tag and have significant higher forward link capability providing a greater range than that of purely passive tags.
In general, the read range of typical passive RFID systems is limited to a few meters. In principal, the antenna size and power of the RF field of the reader can be increased arbitrarily. This will increase the range for transmitting energy to passive tags and will increase the read range somewhat by increasing the sensitivity of the readers' antenna. Recognized by the inventors, however, is that even under ideal conditions, only approximately 30 meters would be achievable. Ideal conditions, however, are seldom the norm. Also recognized by the inventors is that such arbitrary scaling on the transponder side would not generally be possible for tags that would be required to fit through open hydraulic fractures, and thus, would face significant size limitations, especially in applications where the form factor is especially constrained. To fully map hydraulic fractures a read range on the order of 100 meters or so is required. Accordingly, recognized by the inventors is the need for methods and systems which provide transponders or tags that are small enough to be deployed through open or opening hydraulic fractures and which have a communication range with a reader-interrogator of up to 100 meters or more when deployed within a hydraulic fracture of a reservoir.